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The Dust Environment of Comet 67P/Churyumov-Gerasimenko

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The Dust Environment of Comet 67P/Churyumov-Gerasimenko A&A 422, 357–368 (2004) Astronomy DOI: 10.1051/0004-6361:20035806 & c ESO 2004 Astrophysics The dust environment of comet 67P/Churyumov-Gerasimenko M. Fulle1, C. Barbieri2,G.Cremonese3,H.Rauer4, M. Weiler4, G. Milani5, and R. Ligustri6 1 INAF - Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy 2 Dipartimento di Astronomia, Universit`a di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy 3 INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy 4 Institute of Planetary Research, DLR Rutherfordstrasse 2, 12849 Berlin, Germany 5 Unione Astrofili Italiani, UAI 6 Osservatorio di Talmassons, CAST, Talmassons (Ud), Italy Received 4 December 2003 / Accepted 6 April 2004 Abstract. The new target of the ESA Rosetta Mission is comet 67P/Churyumov-Gerasimenko, which passed its last perihelion on 18 August 2002 and was well observed from fall 2002 to spring 2003. Its most prominent feature was a thin dust tail, which is best fitted by the Neck-Line model. Fits of the whole tail provide the dust environment of 67P during a year around perihelion; it shows a strong asymmetry between pre and post perihelion times. The dust mass loss rate appears constant since 2 AU before perihelion at about 200 kg s−1, a factor 100 higher than 46P/Wirtanen, the previous Rosetta target. Neck-Line photometry during 2002 and 2003 suggests that such a dust environment has remained similar since 3.6 AU before perihelion, i.e. the distance at which Rosetta science operations will start and the lander will be delivered to the surface. Key words. space vehicles – comets: general – comets: individual: 67P/Churyumov-Gerasimenko 1. Introduction observability after the perihelion passage of August 18, 2002: before August 2002 and after spring 2003, the comet was too Due to a launch delay, in February 2003 ESA changed the tar- close to the Sun to be observable. We will analyse in another get of the Rosetta Mission (ESA 2003): from 46P/Wirtanen paper the data collected before the 1996 perihelion passage. to 67P/Churyumov-Gerasimenko (67P hereafter for brevity). During 2002 and 2003, 67P showed a 10 arcmin long and thin A strong effort was required to obtain information on the new tail (Fig. 1), bright enough to be monitored by CCD cameras of target in order to change the orbiting strategies to adapt the al- amateur astronomers. This is unusual for short-period comets, ready built spacecraft and instruments to an environment pos- which rarely show bright tails. We will show in this paper that, sibly different from the planned one: this regards not only the certainly during all spring 2003, and probably also during fall shape and size of the nucleus, but also the coma environment, 2002, this tail was a dust tail. If ground-based observations only which is related to shape and spin state of the nucleus. Here we are available, models of dust tails provide the most complete focus on the dust environment of 67P: a comparison with the information on a comet dust environment (Fulle 1999). Coma environment of the previous target 46P will allow us to outline photometry, quantified by the quantity Afρ (A’Hearn et al. the differences between two well studied short-period comets, 1984), will then be used to constrain the outputs of dust tail in order to understand if 67P is representative of the family of models. These suggest that the dust environment of 67P may short-period comets. Moreover, a model of the dust environ- be different from that of typical short-period comets, in which ment is necessary to plan the operations of the many instru- such a long lasting dust tail was never observed. Therefore, a ments studying the dust ejected from the comet, to determine deep analysis of the dust environment of 67P is even more im- the orbiting strategies, the software robustness of the naviga- portant in the perspective of the Rosetta Mission. tion cameras against false nucleus detection, and the lifetime Let us recall that one of the difficulties in interpreting dust of the landing probe against dust pollution. images is that the dust ejections occurring days before, months In this paper we consider IRAS observations performed before or years before the time of observation form patterns in 1983 (Sykes & Walker 1992) and ground-based data col- which may have a similar appearance. Therefore, extreme care lected on the dust ejected from 67P during the nine months of must be paid to obtain a correct identification. If an observed Send offprint requests to: M. Fulle, e-mail: [email protected] pattern is improperly identified, the derived time (and in gen- Based also on observations collected at the National Galileo eral the rate) of dust production will be severely in error. In Telescope and the 2 m telescope of the Th¨uringer Landessternwarte the case of 67P, the identification of the thin tail in terms of Tautenburg. a trail (Sykes & Walker 1992) or in terms of a Neck-Line Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20035806 358 M. Fulle et al.: The dust environment of comet 67P/Churyumov-Gerasimenko Table 1. The observed Position Angle PAT of the Tail is compared with the antisolar direction (Position Angle PARV of the prolonged radius vec- tor) and the expected position of the dust trail (Position Angle PACO of the comet orbit projected on the sky). The observed PAT values are best fit by the computed PANL of the Neck-Line ejected at Sun-Comet distances r(θ−π)andatthet(θ−π)−T times (days with respect to perihelion T), where θ is the comet true anomaly at observation. Observers: DT, Diego Tirelli; RL, Rolando Ligustri (Talmassons Observatory); HM, Hermann Mikuz (Crni Vrh Observatory); TNG, National Galileo Telescope; TLS, Tautenburg Schmidt Telescope at Th¨uringer LandesSternwarte. Time UT t(θ) − Tr(θ) θ t(θ − π) − Tr(θ − π)PANL PA CO PA RV PA T Band Observer [days] [AU] [◦] [days] [AU] [◦][◦][◦][◦] 14.094 Aug. 2002 –4.219 1.29 –3.6 – – – 263.7 272.4 270 ± 3 R RL 8.107 Sep. 2002 21.794 1.32 18.4 –810 5.28 275.4 274.6 283.3 280 ± 3 V DT 19.134 Oct. 2002 62.821 1.49 48.4 –378 3.62 289.8 288.6 293.5 290 ± 3 V DT 8.059 Nov. 2002 82.746 1.61 60.7 –285 3.05 294.2 293.9 295.0 294 ± 3 R RL 11.154 Nov. 2002 85.841 1.63 62.4 –275 2.99 294.7 294.6 295.1 296 ± 3 V DT 4.175 Jan. 2003 140.862 2.03 86.3 –164.2 2.20 299.3 303.9 291.0 296 ± 3 V DT 11.188 Jan. 2003 147.875 2.08 88.7 –156.4 2.14 299.3 304.3 289.6 297 ± 3 R RL 12.197 Jan. 2003 148.884 2.09 89.0 –155.3 2.13 299.3 304.4 289.4 295 ± 3 R RL 2.003 Feb. 2003 167.690 2.23 94.8 –138.0 2.00 298.6 303.9 282.7 297 ± 3 R RL 22.935 Mar. 2003 216.622 2.58 107.0 –107.7 1.78 294.6 297.6 138.7 296 ± 2 R HM 27.000 Mar. 2003 220.687 2.61 107.8 –105.8 1.77 294.4 297.1 133.2 294.5 ± 0.5 R TNG 27.875 Mar. 2003 221.562 2.62 108.0 –105.4 1.76 294.3 297.0 132.2 294.0 ± 0.5 R TLS 28.028 Mar. 2003 221.715 2.62 108.1 –105.3 1.76 294.3 297.0 132.0 294.0 ± 0.5 R TLS 28.896 Mar. 2003 222.583 2.63 108.2 –104.9 1.76 294.3 296.9 131.1 294.0 ± 0.5 R TLS 6.922 Apr. 2003 230.609 2.69 110.1 –101.0 1.73 294.0 296.2 124.5 294 ± 2 R HM 2. 67P Tail: Evolution over nine months Databases of the Comet Section of the Italian Amateur Astronomer Union (UAI) contain many CCD images following the evolution of the thin tail during the whole 2002/03 observ- ability window (UAI 2003). These images are filtered using broadband filters only, like Cousins R, and are here comple- mented by four images of much higher quality obtained at the National Galileo Telescope and the Tautenburg Schmidt Telescope (Weiler et al. 2004a, Fig. 1). The log of all the ob- servations analysed in this paper is reported in Table 1. Here we report also the correspondence among UT times, the comet true anomaly θ (i.e. the angle between the Sun-Comet and the Sun-perihelion vectors), the observation time related to perihe- Fig. 1. Image of the thin tail of 67P taken on 27.875 March 2003 UT lion t(θ) − T, and the Sun-Comet distance r(θ): in the following at the 2 m Tautenburg Schmidt Telescope (see Table 1 for details of sections and figures, Table 1 can be used to convert one of these the observations). 67P showed a similar tail during nine months af- ter 2002 perihelion. The axis units are pixels, with scale 1.23 arc- quantities to the others. sec pixel−1 (the scale reported in Weiler et al. (2004a) is wrong). The The 67P tail is so thin that it can be described as a image width is 9×105 km projected at the Earth-comet distance.
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